Problems associated with anthropogenic climate change have driven the need for increased energy production from non-fossil fuel sources. Geothermal is particularly attractive. A relatively untapped and sustainable energy resource is low temperature geothermal heat. This is heat within the earth's crust that is near enough to the surface to be extracted as hot water or steam. While high grade geothermal heating sources such as seen in the Yellowstone basin are relatively rare, lower grade resources are relatively common along fault lines and around dormant volcanoes, and potentially could operate for thousands of years with good stewardship.
The capacity to generate power from low grade geothermal sources (some of which are below 100° C.) has only recently been developed and benefits from a thermal boost obtained from solar collectors (or other supplementing heat sources) mounted in series or in parallel with the geothermal feed. The state of the art can be appreciated from publications such as by Boyd (Technical Assessment of the Combined Heat and Power Plant at Oregon Institute of Technology, Klamath Falls, Oreg.), as was presented at the Geothermal Resources Council annual meeting of 2012, and by Kuyumcu (Hybrid Geothermal and Solar Thermal Power Plant Case study), presented at the same symposium, both of which describe working power plants based on geothermal and solar organic Rankine Cycle (ORC) technology. Combined concentrated solar and geothermal power generation was also described by Nelson (Concentrated Solar and Geothermal Hybrid Power Project) at the GRC 2012 annual meeting. An early vision of the general concept of combined geothermal and solar power is found in U.S. Pat. No. 4,099,381 to Rappaport. These publications are incorporated in full by reference for all that they teach and disclose.
Thus, combined solar and geothermal heat engines enable use of lower grade geothermal heat sources and represent an exciting form of alternative energy to fossil fuels. A geothermal power cogeneration plant is located at the Oregon Institute of Technology in Klamath Falls, Oreg., and is currently rated for 280 KWe, supplying power and most of the heat to an entire campus. Another plant operates in Turkey at 4-6 MW (depending on the season). The potential energy in known low to moderate temperature geothermal fields is typically discussed in gigawatts of renewable capacity, a vast and widely available resource that may be operated with a negligible carbon footprint.
In a single-pass system, brine extracted from a geothermal field becomes a waste product, and efforts are underway to recycle this brine by cycling it back through the porous subterranean features in a continuous loop. This entails pumping it out from a supply well and back into the geothermal field at a return injection well, where the two wells are connected by a percolating bed in the rock overlying the hot magma. This also reduces the risk of pumping the geothermal feature dry. Thus combined solar/geothermal plants offer sustainable energy with little or no environmental impact.
However, it has not generally been recognized that while solar heating is diurnal, at peak intensities it has the capacity to “back out” the relatively constant geothermal heat input into an ORC boiler, and the solar overheat will result in excess heat being sent back to the return well, such that the capacity to percolate fluid through the subterranean geology can be permanently damaged through chemical processes that affect the Ksp of minerals in the brine and by other degradative processes.
Therefore, there is a pressing need to develop means to more efficiently cool solar-heated geothermal brines before returning them to the earth, and desirably, to increase the use of waste heat from Rankine Cycle devices generally. Methods known for increasing the efficiency of a Rankine Cycle include: lowering the operating pressure—temperature of the condenser by lower the temperature at which heat is rejected (subject to the limiting ambient conditions), adding a superheat to the working vapor, and increasing the evaporator or boiler temperature by adding supplemental heat (and thus increasing the pressure for the same working fluid vapor). It is also an object of this invention to improve efficiency in the heat sink downstream from the boiler and to combine the condenser with a geothermal cooling loop to reduce brine temperature before return to the geothermal field. Also recognized is a need to synchronize power generation with demand. These and other disadvantages of the prior art are addressed by the invention disclosed here.